The present invention relates to a semiconductor memory. More particularly, the present invention relates to a nonvolatile analog memory which can store analog information.
Digital memories have been widely used as information storage media because of their fast random access and small, light, and simple controllers. Such digital memories store either a logic "0" or "1" in a memory cell. In order to increase the storage capability for a given physical dimension of the memory, the size of an individual memory cell must be reduced. However, the reduction of a cell size is limited in view of the current state of manufacturing technology.
Currently, 64 Mbit digital memories and 256 Mbit digital memories are under mass-production, and 1 Gbit and 4 Gbit digital memories are being experimentally manufactured. Using the current manufacturing technology, however, it is difficult to manufacture a digital memory having more than 4 Gbit storage capacity.
In order to overcome the limitation of the technical storage capacity of the digital memory, research into an analog memory which can store multiple values instead of binary values in a memory cell is currently under way. For example, conventional analog memories are disclosed in L. R. Carley, "Trimming analog circuit using floating-gate analog MOS memory," IEEE J. Solid-state circuits, Vol. 24, pp. 1569-1575, (December 1989), T. Blyth, et al., "A non-volatile analog storage device using EEPROM technology" J. ISSCC Digest of Technical Papers, pp. 192-193, (February, 1991), T. Blyth, et al., "An analog trimming circuit based on a floating-gate device", IEEE Electron Device Letters, Vol. 23, pp. 1437-1440, (December 1988).
The conventional analog memory has inherent drawbacks, however, in that it requires a special fabrication process with an ultra thin oxide. In addition, conventional analog memories use inefficient erasing and programming operations.